2018 California Senate - Open Primary (Feinstein vs. de Leon)

From the Civil War until the 1950s, California was solidly Republican.  But the Republicans who were elected were not Republicans in the modern sense of the word. Instead, like many party members in the West at the time, they hewed to a Progressive line. After the 1950s, Progressive Republicans in the northern portion of the state became Democrats, while conservative Democrats and suburbanites were switching and making the southern reaches of the state more Republican.

Today, California's political divide is more of an east-west split than a north-south one. Democrats represent the coastal areas, while Republicans dominate inland. Overall, the state has become solidly Democratic, in large measure due to the transformation of the suburbs into swing areas during the Bill Clinton years.

The prime beneficiaries of this were California's two Democratic senators who were elected along with Clinton in 1992. The younger of the pair, Barbara Boxer, retired in 2016, leading to speculation that Dianne Feinstein, the oldest member of the Senate, would retire as well.  But Feinstein opted run for a fifth full term.  If successful, she will surpass Hiram Johnson and become the longest-serving Senator in California history.  It seems more likely that she will lose a primary challenge against state Senate president Kevin de Leon than the general election.